feat: update API routers, dashboard, graph panel, and gap table UI

Add Langfuse observability via LangChain CallbackHandler

feat(pipeline): RAG quality gate, live coverage, streaming API, recursion limit

fix(agent): rank gaps by score, fuzzy topic filter, stream in web API

feat(agent): implement comprehensive logging and fix recursion loop

feat(api): implement FastAPI read-side layer with 4 endpoints and OpenAPI contract